This invention relates to reception of electromagnetic signals by an array of antenna elements connecting with respective receiving circuits and, more particularly, to the use of optical fibers for communicating received signals and for energizing the receiving circuits.
An array antenna, such as a two-dimensional array having numerous radiators arranged in rows and in columns, may be employed in situations wherein the shape of the surface of the antenna must conform to an underlying support, such as the fuselage or wing of an aircraft. Such construction, heretofore, has been laborious because the support structure which holds the radiators must be configured to fit the underlying support.
By way of example, in the situation where the antenna is formed of a set of radiators imprinted, possibly by photolithography, upon a substrate, the substrate must be built to fit the underlying support. The signals radiated and/or received by the radiators may be phase shifted, and may be provided with an amplitude taper so as to compensate for curvature in the underlying support. The structure of the antenna may be complicated by the need for multiple receiving circuits connected directly to respective ones of the radiators so as to avoid excessive signal attenuation as might otherwise develop in the communication of a received signal from a radiator to a distant receiving circuit. As an additional complicating factor, there is a difficulty in locating a multitude of wires providing for communication of signal, control, and power to the various receiving circuits.
As a further example in the deployment of an array antenna, such an antenna may be deployed by a satellite circling the earth. In such case, a rigid antenna, heretofore, has been fabricated of sections which articulate relative to each other, thereby to permit stowage on board the spacecraft which is to deploy the antenna. Such construction does not permit the use of a continuous antenna without points of articulation. In addition, the mechanical structure needed to provide for the articulation increase the weight and the complexity of the antenna. It is noted also, that in the case of the antenna carried by the spacecraft, it may be desired to construct the antenna as a series of radiators radiating in both forward and reverse direction, such an antenna being comprised of, by way of example, a set of radiators disposed on an electrically insulating substrate without use of a reflective plane. With such construction, the numerous wires interconnecting the various radiators with a beam former can act as a metallic screen which reflects radiation and, thereby, would alter the radiation pattern of the antenna.
The aforementioned disadvantages are overcome and other advantages are provided by an array antenna constructed in accordance with the invention wherein the radiators, such as dipole radiators, are disposed on a flexible sheet of electrically-insulating material. This construction enables the antenna to be placed on an underlying support which has a curved surface, such as the aforementioned fuselage or airfoil, by way of example. In addition, the flexibility of the antenna enables the antenna to be rolled into a long cylinder, by way of example, for stowage on board a spacecraft for later deployment in a planar or curved configuration, this being accomplished without the aforementioned points of articulation. Thus, a single construction of antenna can be employed to overcome the above-noted disadvantages of antennas to be deployed by spacecraft and by antennas to be borne by vehicles.
In a preferred embodiment of the invention, receiving circuits are coupled to the radiators, the coupling occurring directly at the substrate to minimize length of interconnecting electric wires between the radiators and their respective receiving circuits. In accordance with an important feature of the invention, fiber optic cables are provided for interconnecting signals outputted by the receiving circuits to a beam former, which beam former may be located at a point distant from the antenna, if desired. The individual optical fibers which communicate the received signals are free of any metallic, electrically-conducting material so as to avoid the aforementioned disadvantage of reflecting radiant energy, thereby to avoid distortion of the radiation pattern of the antenna. In addition, in accordance with a further feature of the invention, electric power for operating the circuitry in each of the receiving circuits is provided by optically transmitting power from a laser power source. The optical power is carried by an optical fiber and is converted to electric power at each of the respective receiving circuits.
In each of the receiving circuits, there is a photo cell which converts optical power of the laser, received by the optical fiber, to electrical power for operation of an IF (intermediate frequency) circuit to convert an input RF (radio frequency) signal to an IF signal, and also to provide power for operation of an optical modulator assembly upon rays of light obtained from a laser. The optical modulator assembly converts the electrical IF signal to an optical signal wherein a beam of light is modulated in amplitude by the IF signal to provide the optical output signal of the receiving circuit.
In accordance with a further feature of the invention, each receiving circuit is constructed with flexibility to allow for a flexing of the circuit concurrent upon a flexing of the antenna substrate. The flexibility of the receiving circuit is attained by constructing the receiving circuit of individual modules connected by flexible optical cable. In a preferred embodiment of the invention, each receiving circuit comprises three of the modules, the three modules being interconnected by two flexible junctions. Each of the modules itself is rigid and is constructed of discrete analog components supported on a printed circuit board. The modules include components such as the mixer, the photo cells, a photodetector for receiving an optical bias signal as well as an optical calibration signal, and the optical modulator assembly with its included laser diode. At a junction between two of the modules, supporting structure is provided at each of the modules for engagement with the interconnecting optical cable. The entire set of three modules constituting a single receiving circuit is encased with plastic film, such as shrink-wrap film which is electrically insulating. The film serves as a housing for providing dimensional stability to the assembly of the three modules, while allowing for flexing between the modules at the junction points.
In accordance with yet a further feature of the invention, in each of the receiving circuits, the three modules are connected serially to give a configuration similar to that of a pen. The length of the receiving circuit is less than the spacing between two successive ones of the radiators in a row of the radiators in the array of the antenna. Thereby, the successive receiving circuits can be arranged in the manner of the cars of a train, thereby to extend along a row of radiators of the antenna. Successive rows of the receiving circuits are employed for successive ones of the rows of the radiators in the antenna array.
In order to facilitate wiring by the optical fibers among the various receiving circuits within the array, each of the receiving circuits is provided with a set of multiple optical fibers which include a sufficient number of fibers to service all of the receiving circuits within a single row, with respect to their electric power and their signals. By way of example, if there are 25 receiving circuits in a single row, 25 of the optical fibers which have been set aside for input signals of the receiving circuits are employed in the first of the receiving circuits, Correspondingly, only 24 of this set of optical fibers are employed in the second of the receiving circuits, with 23 of the fibers being employed in the third of the receiving circuits, with corresponding reduction in the number of used optical fibers in the successive ones of the receiving circuits in the row of receiving circuits. This permits all of the receiving circuits to be fabricated with the same construction, only the interconnection of specific ones of the fibers differs among the respective receiving circuits in the row. This provides for simplicity in the physical arrangement of the components of the antenna and facilitates the construction while ensuring greater reliability in the use of the antenna even during a flexing of the antenna It is noted that the capacity for the receiving circuits to flex enables the antenna to flex without interference from the receiving circuits.